1. Technical Field
The invention relates to a receiver, a signal processing method and a program.
2. Description of the Related Art
Recently, various electronic apparatuses, such as car navigation apparatuses, cell phones and digital still cameras, are provided with a positioning function using a GPS (Global Positioning System). Typically, when an electronic apparatus uses a GPS, a GPS module receives a signal from four or more GPS satellites to measure the position of the apparatus based on the received signal, and the measurement result is notified to users through the screen or the like of a display apparatus. In more detail, the GPS module demodulates the received signal to acquire orbital data of each GPS satellite, and derives the three-dimensional position of the apparatus from the orbital data, time information and delay time of the received signal by using a simultaneous equation. The reason that four or more GPS satellites, from which the signal is transmitted, are necessary is for eliminating the influence of an error between internal time of the module and time of the satellites.
Herein, the signal (L1 band, C/A codes) transmitted from the GPS satellites is obtained by BPSK (Binary Phase Shift Keying)-modulating a spectrum spread signal, which is obtained by spectrum-spreading data of 50 bps by using Gold codes. The Gold code is a kind of a pseudo-random code with a code length of 1023 and a chip rate of 1.023 MHz. Further, for the BPSK modulation, a carrier of 1575.42 MHz is used. Thus, when the GPS module receives the signal from the GPS satellites, it is necessary to synchronize the spreading code, the carrier and the data.
In general, a GPS module mounted on an electronic apparatus frequency-converts a carrier frequency of a received signal into an IF (Intermediate Frequency) of several MHz or less, and then performs the above-described synchronization process and the like (for example, refer to Japanese Unexamined Patent Application Publication No. 2003-232844). A typical IF, for example, is 4.092 MHz, 1.023 MHz, 0 Hz or the like. Normally, a received signal has a signal strength smaller than signal strength of a thermal noise, and a S/N less than 0 dB. However, the signal can be demodulated by a spread spectrum process gain. In the case of a GPS signal, for example, a process gain with respect to a data length of 1 bit is 10 Log(1.023 MHz/50)≈43 dB.